1. Structural Analysis 7th Edition in SI Units
Chapter 3:
Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition in SI Units
Russell C. Hibbeler

2. Common Types of Trusses
A truss is a structure composed of slender members joined together at their end points
The joint connections are usually formed by bolting or welding the ends of the members to a common plate called gusset
Planar trusses lie in a single plane & is often used to support roof or bridges
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

3. Common Types of Trusses
Roof Trusses
They are often used as part of an industrial building frame
Roof load is transmitted to the truss at the joints by means of a series of purlins
To keep the frame rigid & thereby capable of resisting horizontal wind forces, knee braces are sometimes used at the supporting column
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

4. Common Types of Trusses
Roof Trusses
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

5. Common Types of Trusses
Bridge Trusses
The load on the deck is first transmitted to the stringers -> floor beams -> joints of supporting side truss
The top & bottom cords of these side trusses are connected by top & bottom lateral bracing resisting lateral forces
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

6. Common Types of Trusses
Bridge Trusses
Additional stability is provided by the portal & sway bracing
In the case of a long span truss, a roller is provided at one end for thermal expansion
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

7. Common Types of Trusses
Assumptions for Design
The members are joined together by smooth pins
All loadings are applied at the joints
Due to the 2 assumptions, each truss member acts as an axial force member
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

8. Classification of Coplanar Trusses
Simple , Compound or Complex Truss
Simple Truss
To prevent collapse, the framework of a truss must be rigid
The simplest framework that is rigid or stable is a triangle
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

9. Classification of Coplanar Trusses
Simple Truss
A simple truss is the basic “stable” triangle element is ABC
The remainder of the joints D, E & F are established in alphabetical sequence
Simple trusses do not have to consist entirely of triangles
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

10. Classification of Coplanar Trusses
Compound Truss
It is formed by connecting 2 or more simple truss together
Often, this type of truss is used to support loads acting over a larger span as it is cheaper to construct a lighter compound truss than a heavier simple truss
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

11. Classification of Coplanar Trusses
Compound Truss
Type 1
The trusses may be connected by a common joint & bar
Type 2
The trusses may be joined by 3 bars
Type 3
The trusses may be joined where bars of a large simple truss, called the main truss, have been substituted by simple truss, called secondary trusses
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

12. Classification of Coplanar Trusses
Compound Truss
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

13. Classification of Coplanar Trusses
Complex Truss
A complex truss is one that cannot be classified as being either simple or compound
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

14. Classification of Coplanar Trusses
Determinacy
Total unknowns = forces in b no. of bars of the truss + total no. of external support reactions
Force system at each joint is coplanar & concurrent
Rotational or moment equilibrium is automatically satisfied
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

15. Classification of Coplanar Trusses
Determinacy
Therefore only
By comparing the total unknowns with the total no. of available equilibrium eqn, we have:
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

16. Classification of Coplanar Trusses
Stability
If b + r < 2j => collapse
A truss can be unstable if it is statically determinate or statically indeterminate
Stability will have to be determined either through inspection or by force analysis
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

17. Classification of Coplanar Trusses
Stability
External Stability
A structure is externally unstable if all of its reactions are concurrent or parallel
The trusses are externally unstable since the support reactions have lines of action that are either concurrent or parallel
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

18. Classification of Coplanar Trusses
Internal Stability
The internal stability can be checked by careful inspection of the arrangement of its members
If it can be determined that each joint is held fixed so that it cannot move in a “rigid body” sense wrt the other joints, then the truss will be stable
A simple truss will always be internally stable
If a truss is constructed so that it does not hold its joints in a fixed position, it will be unstable or have a “critical form”
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

19. Classification of Coplanar Trusses
Internal Stability
To determine the internal stability of a compound truss, it is necessary to identify the way in which the simple truss are connected together
The truss shown is unstable since the inner simple truss ABC is connected to DEF using 3 bars which are concurrent at point O
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

20. Classification of Coplanar Trusses
Internal Stability
Thus an external load can be applied at A, B or C & cause the truss to rotate slightly
For complex truss, it may not be possible to tell by inspection if it is stable
The instability of any form of truss may also be noticed by using a computer to solve the 2j simultaneous eqns for the joints of the truss
If inconsistent results are obtained, the truss is unstable or have a critical form
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

21. Example 3.1
Classify each of the trusses as stable, unstable, statically determinate or statically indeterminate. The trusses are subjected to arbitrary external loadings that are assumed to be known & can act anywhere on the trusses.
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

22. Solution For (a), Externally stable
Reactions are not concurrent or parallel
b = 19, r = 3, j = 11
b + r =2j = 22
Truss is statically determinate
By inspection, the truss is internally stable
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

23. Solution For (b), Externally stable b = 15, r = 4, j = 9
Truss is statically indeterminate
By inspection, the truss is internally stable
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

24. Solution For (c), Externally stable b = 9, r = 3, j = 6
Truss is statically determinate
By inspection, the truss is internally stable
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

25. Solution For (d), Externally stable b = 12, r = 3, j = 8
The truss is internally unstable
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

26. The Method of Joints
Satisfying the equilibrium eqns for the forces exerted on the pin at each joint of the truss
Applications of eqns yields 2 algebraic eqns that can be solved for the 2 unknowns
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

27. The Method of Joints
Always assume the unknown member forces acting on the joint’s free body diagram to be in tension
Numerical solution of the equilibrium eqns will yield positive scalars for members in tension & negative for those in compression
The correct sense of direction of an unknown member force can in many cases be determined by inspection
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

28. The Method of Joints
A +ve answer indicates that the sense is correct, whereas a –ve answer indicates that the sense shown on the free-body diagram must be reversed
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

29. Example 3.2
Determine the force in each member of the roof truss as shown. State whether the members are in tension or compression. The reactions at the supports are given as shown.
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

30. Solution
Only the forces in half the members have to be determined as the truss is symmetric wrt both loading & geometry,
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

31. Solution Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

32. Solution Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

33. Zero-Force Members
Truss analysis using method of joints is greatly simplified if one is able to first determine those members that support no loading
These zero-force members may be necessary for the stability of the truss during construction & to provide support if the applied loading is changed
The zero-force members of a truss can generally be determined by inspection of the joints & they occur in 2 cases.
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

34. Zero-Force Members Case 1
The 2 members at joint C are connected together at a right angle & there is no external load on the joint
The free-body diagram of joint C indicates that the force in each member must be zero in order to maintain equilibrium
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

35. Zero-Force Members Case 2
Zero-force members also occur at joints having a geometry as joint D
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

36. Zero-Force Members Case 2
No external load acts on the joint, so a force summation in the y-direction which is perpendicular to the 2 collinear members requires that FDF = 0
Using this result, FC is also a zero-force member, as indicated by the force analysis of joint F
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

37. Example 3.4
Using the method of joints, indicate all the members of the truss that have zero force.
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

38. Solution
We have,
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

39. Solution Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

40. The Method of Sections
If the forces in only a few members of a truss are to be found, the method of sections generally provide the most direct means of obtaining these forces
This method consists of passing an imaginary section through the truss, thus cutting it into 2 parts
Provided the entire truss is in equilibrium, each of the 2 parts must also be in equilibrium
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

41. The Method of Sections
The 3 eqns of equilibrium may be applied to either one of these 2 parts to determine the member forces at the “cut section”
A decision must be made as to how to “cut” the truss
In general, the section should pass through not more than 3 members in which the forces are unknown
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

42. The Method of Sections
If the force in GC is to be determined, section aa will be appropriate
Also, the member forces acting on one part of the truss are equal but opposite
The 3 unknown member forces, FBC, FGC & FGF can be obtained by applying the 3 equilibrium eqns
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

43. The Method of Sections
When applying the equilibrium eqns, consider ways of writing the eqns to yield a direct solution for each of the unknown, rather than to solve simultaneous eqns
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

44. Example 3.5
Determine the force in members CF and GC of the roof truss. State whether the members are in tension or compression. The reactions at the supports have been calculated.
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

45. Solution
The free-body diagram of member CF can be obtained by considering the section aa,
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

46. Solution
The free-body diagram of member GC can be obtained by considering the section bb,
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

47. Example 3.6
Determine the force in member GF and GD of the truss. State whether the members are in tension or compression. The reactions at the supports have been calculated.
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

48. Solution
The distance EO can be determined by proportional triangles or realizing that member GF drops vertically 4.5 – 3 = 1.5m in 3m.
Hence, to drop 4.5m from G the distance from C to O must be 9m
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

49. Solution The angles FGD and FGF make with the horizontal are
tan-1(4.5/3) = 56.3o
tan-1(4.5/9) = 26.6o
Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd

50. Solution Chapter 3: Analysis of Statically Determinate Trusses
Structural Analysis 7th Edition
© 2009 Pearson Education South Asia Pte Ltd